The present invention relates to a method of producing a semiconductor device, and particularly to a method for solving a problem of dimensional losses in case of performing hole processing using as a mask a resist pattern with its cross-sectional shape inversely tapered.
As design rules of semiconductor devices have become minute as seen in VLSI and ULSI, studies have been made on high numerical aperture of an exposure device, short wavelength of exposure and improvement of photoresist materials in the field of photolithography. Particularly, excimer laser lithography using an excimer laser light source such as a KrF excimer laser light (248 nm) instead of a conventional g line (436 nm) or i line (365 nm) of high pressure mercury lamp as an exposure light source is noted as an art whereby high resolution may be attained relatively easily.
Meanwhile, in excimer laser lithography, it is difficult to apply a novolac based positive photoresist, which has been typically used for the g-line exposure and i-line exposure. A novolac resin has a base resin and an aromatic ring of naphthoquinone diazide based compound added as a photosensitizer have large absorption in a wavelength range of the KrF excimer laser light. Thus, the novolac based positive photoresist lacks sensitivity. In addition, transmittance of the exposure light is drastically reduced, and the cross-sectional shape of the photoresist pattern is tapered.
Also, in a stepper using the excimer laser as the light source, the laser light of a narrow range of wavelengths is used for the purpose of eliminating chromatic aberration due to wavelength distribution. Thus, in order to cover the shortage of exposure, a material of high sensitivity is demanded as a resist material.
In view of the above-described status of the art, a photoresist material whereby high sensitivity and high resolution can be attained at the excimer laser wavelength is demanded. As such a photoresist, so-called chemical amplification resist has been noted recently. The chemical amplification resist is a photoresist of a type for generating acid by photoreaction from a photoreactive acid generator such as onium salt or a polyhalogen compound, referred to hereinafter simply as a photo-acid generator, and then advancing the resist reaction such as polymerization, crosslinking and conversion of functional groups by thermal processing or post-baking in the presence of the acid, thereby generating changes in melting speed.
Chemical amplification resists are categorized into negative and positive types in accordance with the type of resist reaction, and into two-component system, three-component system and the like in accordance with the number of basic components. Under the status quo, a negative three-component resist which uses novolac resin for the base resin, DDT or p,p'-dichlorodiphenyltrichloroethane for the photo-acid generator, and hexamethylolmelamine for the acid crosslinking agent is regarded as most practical. The resolution mechanism of this resist is based on the fact that acid is generated from DDT by exposure of the KrF excimer laser, and then promotes crosslinking of the base resin due to the crosslinking agent at the time of post-baking, thereby rendering an exposure portion insoluble in an alkali.
However, the chemical amplification resist has a problem that a large gradient is generated if material design to raise absorptance is carried out in order to reduce standing-wave effects. That is, in the positive resist, acid diffusion becomes great in the vicinity of the surface of the resist layer, promoting a light decomposition reaction, so that the cross-sectional shape of the resist pattern after development is tapered. On the contrary, in the negative resist, the crosslinking reaction is promoted in the vicinity of the surface of the resist layer, so that the cross-sectional shape after development is inversely tapered. If hole processing for opening a contact hole or a via-hole, for instance, is carried out using as a mask the resist pattern with the cross-sectional shape deteriorated in the above-mentioned manner, the following problems are generated. The problems are described referring to FIGS. 1a, 1b and FIGS. 2a, 2b. Partly common numerals are used in these figures.
First, a case is considered in which a surface of an interlayer insulation film 12 stacked on an underlying metallization 11 having a step is flattened with a positive photoresist film, and is patterned to form a resist pattern 13 having its cross-sectional shape tapered, as shown in FIG. 1a. The resist pattern 13 has a first aperture 14 in an upper region of the step and a second aperture 15 in a lower region of the step. However, since the apertures 14, 15 having the same tilt are formed in regions which are different in thickness of the photoresist film, a diameter A.sub.1 of the first aperture 14 on the bottom is larger than a diameter B.sub.1 of the second aperture 15 on the bottom. As etching of the interlayer insulation film 12 in a later process is carried out in a normal ion mode, it is the bottom that prescribes an incidence range of ion. Accordingly, if etching is carried out using the resist pattern 13 as a mask, diameters of contact holes 16, 17 which are formed, will substantially reflect the diameters A.sub.1, B.sub.1 on the bottom of the apertures 14, 15, respectively. In short, even though the contact holes 16, 17 have the same dimensional design above and below the step, different diameters are obtained actually.
Meanwhile, if the surface of the interlayer insulation film 12 is flattened with the negative photoresist film, the cross-sectional shape of the resist pattern 18 after the phenomenon is inversely tapered, as shown in FIG. 2a. At this time, a first aperture 19 and a second aperture 20 are formed in an upper region and a lower region of the step, respectively. However, diameters A.sub.2, B.sub.2 on the uppermost surfaces of the apertures 19, 20 which prescribe an incidence range of ion in etching of interlayer insulation film 12 in a later process are equal. Accordingly, if the interlayer insulation film 12 is etched in the ion mode, diameters A.sub.3, B.sub.3 of contact holes 21, 22 formed to correspond to the apertures 19, 20 becomes equal to each other.
Thus, the problem of the difference in the diameters of the contact holes above and below the step is tentatively solved by masking with the negative resist pattern. In the process under the status quo, etching conditions for securing high selectivity to the resist pattern are employed.
However, these apertures are not necessarily formed as designed by the dimensional design or anisotropically. In the example shown in FIG. 2b for instance, the diameters A.sub.3, B.sub.3 of the contact holes 21, 22 are larger than the diameters A.sub.2, B.sub.2 of the apertures 19, 20. It is considered that such dimensional losses are generated by the enlargement of the diameters due to an attack against sidewalls of the etching pattern by a slant incidence component created by the diffusion of the incident ions at an edge portion of the resist pattern 18. Such a resist pattern which is inversely tapered is positively utilized for taper-etching of a contact hole, as described in the Extended Abstract of the 49th Autumn Meeting of the Japan Society of Applied Physics, Lecture No. 7p-K-14, p. 567. Thus, it is essentially difficult to apply the inversely tapered resist pattern to anisotropic processing.